Heidelberg Catechism, Lord’s Day 41
And, as ever, the Catechism gives reasons, not just imperatives. God hates unchastity (which, we should note, does not mean "sexual activity" but "sexual activity outside of marriage"), so we should "thoroughly detest it", whether we are married or not. "We are temples of the Holy Spirit, body and soul, and God wants both to be kept clean and holy." Consequently, we should not only abstain from sexual immorality; we should abstain from anything that might cause it (actions, looks, talk, thoughts, desires), and from anything that might cause something that might cause it (pornography is an obvious example, but to be honest, so are an awful lot of adverts, TV programmes, conversations and jokes). Yes, that's a big ask. No, it isn't easy. But that's why the gospel, so beautifully expounded in the first half of the Catechism, is so vital.]
Q108. What does the seventh commandment teach us?
A108. That God condemns all unchastity,
and that therefore we should thoroughly detest it
and live decent and chaste lives,
within or outside of the holy state of marriage.
Q109. Does God, in this commandment,
forbid only such scandalous sins as adultery?
A109. We are temples of the Holy Spirit, body and soul,
and God wants both to be kept clean and holy.
That is why God forbids
all unchaste actions, looks, talk, thoughts, or desires,
and whatever may incite someone to them.